1. Field of the Invention
The present invention relates to a so called stripline patch type antenna to be utilized in a microwave communication.
2. Description of the Background Art
Presently, parabola antennas and plane antennas are utilized for the microwave communication. Since the satellite broadcasting has started, although the parabola antennas are more commonly employed for this purpose so far, plane antennas have been attracting much attentions, because plane antennas are thin plate shaped, light weighted, and hence easier to handle.
Up to date, various types of plane antennas have been developed, including a microstrip antenna, a strip patch antenna, a radial line antenna, and a suspended line antenna.
In particular, a type of a plane antenna in which the strip patch antenna is combined with a slot plate is known to be capable of obtaining a high antenna gain.
An example of a stripline patch antenna with a slot plate is shown in FIGS. 1(A) and 1(B). This stripline patch antenna 101 comprises a plate shaped dielectric substrate 102, a grounding conductor plate 103 attached on a back of the dielectric substrate 102, a square shaped radiating element 104 attached on a front of the dielectric substrate 102, a feed line 105 connected to the radiating element 104, and a slot plate 107 having a slot 106 above the radiating element 104, which is mounted at a prescribed distance above the dielectric substrate 102. Although not shown in FIGS. 1(A) and 1(B), the entire antenna is formed from a plurality of radiating element 104 and slot 106 combinations just described.
In this stripline patch antenna 101, when signals to be transmitted are supplied from a transmitter device through the feed line 105, the signals are transformed into radio wave by the radiating element 104, which is then emitted through the slot 106. On the other hand, when the radio wave is received through the slot 106, this radio wave is transformed into signals by the radiating element 104, and the obtained signals are then supplied to a receiver device through the feed line 105.
A relationship between relative antenna gain and an angle for this stripline patch type antenna 101 at 12 GHz frequency is shown in FIG. 2, while a relationship between relative dielectric constant of the dielectric substrate 102 and antenna gain for this stripline patch antenna 101 is shown in FIG. 3. As can be seen from FIG. 3, the antenna gain for this stripline patch antenna is at most 10 dB, but material having a relative dielectric constant of about 2 is normally used, so that the antenna gain is usually about 7 to 8 dB.
This implies that in order to obtain the antenna gain of over 30 dB, which is required for a satellite broadcasting receiver, it is necessary to have 500 to 1000 radiating elements 104 in a single plane antenna.
However, if number of the radiating elements 104 is increased, with ample separation between neighboring radiating elements maintained, then the feed lines 105 have to be lengthened, which in turn increases loss due to the feed lines 105. This is because, in order to obtain the maximum effective signal transmission through the feed lines 105, the impedance of the feed lines 105 has to be adjusted by changing the widths of the feed lines 105, while the feed lines 105 also have to make turns and branches in order to be arranged in the space between the radiating elements 104, and such changing widths and turning and branching of the feed lines 105 are the source of the loss due to the feed lines 105, which will be increased when the feed lines 105 are lengthened.
In a case of the stripline patch antenna 101 which is equipped with the slot plate 107, the feed lines 105 are effectively shielded between the grounding conductor plate 103 and the slot plate 107, so that the loss due to the feed lines 105 is less than that for an antenna without a slot plate. However, in this case, the transmission losses within the feed lines 105 themselves are larger, so that when gain of over 30 dB is to be obtained, efficiency of only about 50 to 60% can be achieved. This is inferior to parabola antenna which can achieve the efficiency of over 70% for the same gain. Consequently, in order to achieve the same high efficiency as the parabola antenna does by the plate antenna, the area of the plane antenna has to be 20 to 40% larger than that of the parabola antenna.
On the other hand, if the separation between the neighboring radiating elements is shortened, the loss due to interference between the neighboring radiating elements 104 or between the radiating elements 104 and the feed lines 105 increases, so that it is difficult to obtain efficiency of over 90% by revising the arrangement, even if the loss due to feed lines 105 themselves is ignored.